Description:
------------
AppendIterator segfault when trying to iterate on a closed Generator

Test script:
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<?php

function createGenerator() { yield 1; }
$generator = createGenerator();

$appendIterator = new AppendIterator();
$appendIterator->append($generator);

iterator_to_array($appendIterator);
try {
    iterator_to_array($appendIterator);
} catch (\Exception $e) {
    echo $e->getMessage();
}


Expected result:
----------------
Cannot traverse an already closed generator


Actual result:
--------------
Segmentation fault